1. Field of the Invention
The present invention relates to a method and a device for producing brushes.
2. Description of the Related Art
DE-pat. 845 933 discloses a method for producing brushes, especially tooth brushes, in a moulding tool. In the case of said known method bristle bundles composed of a plurality of bristle filaments are introduced into channels formed on a moulding tool component of an injection mould. The fixing-side ends of the bristle bundles are partially melted so as to form an enlargement. For this purpose, a hot plate is applied to the fixing-side ends of the bristle bundles, which partially melt thus forming an enlargement protruding beyond the diameter of the bristle bundle. Following this, the mould cavity is closed and thermoplastic material in a flowable state is injected into the moulding tool. According to this known method, the enlargement is embedded in spaced relationship with the outlet opening of the channel in the brush body produced from said thermoplastic material.
Further developments of the method of the type in question were focussed on the sealing of the channels during the mould filling process. DE-A 38 32 520, for example, suggests special sealing measures by means of which the bristle bundles were sealed from the channel wall in cases in which the enlargement was arranged in spaced relationship with the outlet opening of the channel. According to the disclosure of DE-C-35 11 528 the channels should preferably be sealed by positioning the enlargement in contact with said channels.
In addition to the sealing of the channels during the mould filling process, the embedding depth of the enlargement in the brush body, i.e. the distance between the enlargement and the surface of the brush body, is of essential importance. When the enlargement is located directly on the surface of the brush body, the bristle bundles will have a low resistance to being pulled out. Accordingly, DE-C-36 42 124 suggested a method of the type in question in the case of which the channels were extended by a sleeve projecting into the mould cavity so that, on the one hand, the channel was sealed by the enlargement abutting on the sleeve and so that, on the other hand, the enlargement was arranged in spaced relationship with the surface of the brush body.
It follows that the person skilled in the art knows a plurality of methods of the type in question in the case of which the enlargement is arranged at different distances from the surface of the brush body.
EP-B-0 759 711 suggests a method of the type in question in which the enlargement is spaced from the outlet opening of the channel in such a way that, when thermoplastic material is subsequently filled into the injection moulding tool, a thermally weakened length or linear area of the plastic monofilaments, which was created when the fixing-side ends were partially melted and which protrudes beyond the enlargement, is encompassed by the molten plastic material. According to the disclosure of this prior art, the weakened length area of the bristle bundles, which is arranged between the enlargement and the outlet opening, is fully encompassed by the solidified plastic material of the brush body so as to achieve the highest possible bending elasticity and good re-straightening properties of the bristle bundles. This method is, however, disadvantageous insofar as the enlargement must be arranged at a distance from the outlet opening of the channel or at a distance from a sleeve extending the length of said channel; for finding out said distance, the thermally weakened length area has to be determined by complicated measures.
It is the object of the present invention to improve the method according to the generic clause and, in particular, to provide a method by means of which brushes can be produced whose bristle bundles have a good bending elasticity and good re-straightening properties irrespectively of the embedding depth of the enlargement. According to a further aspect, it is the object of the present invention to provide a device which is suitable for executing said method.
In order to achieve the above object as far as the method is concerned, it is suggested that, according to a first aspect of the present invention, a weakened length area of the bristle filaments, which is created when the bristle bundle is partially melted, should be surrounded completely by the enlargement.
According to this first aspect of the present invention, the partial melting of the fixing-side ends of the bristle bundle or bristle bundles is carried out such that the melt formed during said partial melting will partially encompass the non-melted bristle filaments in the longitudinal direction thereof to such an extent that the weakened length area of all bristle filaments will be surrounded by the enlargement, said weakened length area being caused by thermal influences. Practical tests have shown that melt produced on the end face of the bristle bundle can be distributed over the circumference of the non-melted bristle bundle without causing, due to thermal influences, a reorientation of the molecular chains of the oriented filaments in the longitudinal direction of the bristle filaments behind the melt front.
It follows that, according to a first aspect of the present invention, bristle bundles are produced, which are provided with an enlargement on their fixing-side end and the thermally weakened length area of which does not extend beyond said enlargement. Hence, the bristle area of the brush produced has a good bending elasticity and good re-straightening properties independently of the embedding depth of the enlargement in the brush body.
According to another aspect, the present invention further develops the method insofar as hot gas is used for forming the enlargement.
In comparison with the hot knife which is known from the prior art and which is brought into contact with the fixing-side ends of the bristle bundles, the method can be executed particularly economically when the enlargement is formed by means of hot air. In accordance with the method according to the present invention, the bristle bundles are partially melted in a contact-free manner on the fastening side; this will lead to a preferred spherical shape of the enlargement and the clamping force required for holding the bristle filaments in the channels will be small. In addition, when a hot gas is used, the amount of heat applied to the fixing-side end can be controlled more easily than in cases in which a hot knife is used whose temperature is established due to comparatively slow heat conducting processes. The use of hot gas additionally provides the possibility of influencing the propagation direction of the melt through the orientation of the gas jet.
With respect to the optical qualities of the brush, especially in cases where a transparent material is used for the brush body, the gas used should preferably be an inert gas.
The propagation direction of the melt can be influenced in a particularly effective manner with small nozzles. Nozzles having a discharge cross-section which corresponds to 2 to 20 times, preferably 5 to 10 times, the cross-section of the bristle bundle acted upon can be regarded as small nozzles.
With respect to a complete embedding of the thermally weakened length area, the hot gas should preferably be blown against the end face of the bristle bundle. In the case of such a preferred method, the melt created on the end face of the bristle bundle is, by means of the air applied, first urged radially outwards, relative to the bristle bundle, and then pushed partially over the bristle bundle in the longitudinal direction.
In accordance with a preferred embodiment of the method according to the present invention, the fixing-side end of the bristle bundle is arranged in spaced relationship with the moulding tool component, but the enlargement is formed without there being any distance to the channel. In the case of this preferred embodiment, the flow front of the enlargement solidifies at the outlet opening of the channel. The bristle bundles are here held in the channel, i.e. the bristle filaments abut on the comparatively cold moulding tool component. The outer bristle filaments, which come into direct contact with the melt running down the bristle bundle, are cooled by the moulding tool component. This prevents the filaments projecting beyond the enlargement from being heated to a critical temperature at which a thermally caused weakening has to be reckoned with. It turned out that, especially in the case of oriented filaments consisting of polyamide, said critical temperature is substantially equal to the melting temperature.
In accordance with another preferred embodiment of the present invention, the melt forming the enlargement is brought into contact with the moulding tool component. Following this and prior to filling the moulding tool with the flowable mass, the bristle bundle provided with the enlargement is preferably axially displaced, i.e. it is arranged in spaced relationship with the surface of the moulding tool component and, consequently, in spaced relationship with the outlet opening of the channel, so as to achieve the best possible resistance to pulling out.
With respect to an economical execution of the method, the moulding tool component should preferably be moved past a blowing device giving off hot gas so as to form the enlargement, and the moulding tool component should sealingly be installed in the moulding tool so as to complete the same. When the method is executed in this way, the moulding tool component and, consequently, the formation of the enlargement is decoupled from the moulding tool, i.e. the mould filling cycle is independent of the preparation of the bristle bundles (insertion of the bristle filaments in the channels of the moulding tool component, formation of the enlargement, axial displacement, if desired).
In accordance with another preferred embodiment of the present invention, the moulding tool component is cooled while being moved either towards or away from the moulding tool. This kind of cooling is to be preferred especially in combination with the formation of the enlargement without there being any distance to the channel and/or the formation of the enlargement by bringing the melt into contact with the moulding tool component with regard to the best possible cooling of the bristle filaments arranged on the outer circumference of the bristle bundle.
In order to achieve the above object as far as the device is concerned, a device according to the generic clause, which is used for producing brushes, especially toothbrushes, and which comprises a multi-cavity moulding tool, a mould filling means communicating with the moulding tool and a heating means associated with the moulding tool holding the bristle bundles, is further developed in such a way that the heating device comprises at least one hot-gas nozzle which is directed towards the moulding tool component.
Such a device can be controlled precisely and with a short reaction time as far as the heat input at the fixing-side ends of the bristle bundles is concerned. The disadvantages of a heated-knife control determined by slow heat conducting processes are eliminated in this way.